1. Field of the Invention
The present invention relates to an image forming apparatus such as a printer, a fax machine, and a copying machine, employing an electrophotographic system or an electrostatic recording system.
2. Description of the Related Art
Hitherto, there have been known various image forming apparatus employing an electrophotographic system or an electrostatic recording system in an image forming process as described below. As an example thereof, there is given an image forming apparatus of an intermediate transfer tandem system in which a plurality of process cartridges are arranged in a row in a rotational direction of an intermediate transfer belt tensioned rotatably, and a color image is formed via the intermediate transfer belt.
As the image forming apparatus of an intermediate transfer tandem system, there is an image forming apparatus in which an endless intermediate transfer belt (intermediate transfer member) is tensioned by a plurality of tension rollers including a drive roller. This image forming apparatus includes a primary transfer portion formed of a photosensitive drum and a primary transfer roller, which are arranged to be opposed to each other at positions capable of interposing the intermediate transfer belt therebetween in a region between the two tension rollers. This image forming apparatus further includes a secondary transfer portion formed of a secondary transfer inner roller and a secondary transfer outer roller (secondary transfer rotary member), which is capable of being brought into abutment against and being separated from the secondary transfer inner roller. The secondary transfer inner roller and the secondary transfer outer roller are arranged to be opposed to each other so as to interpose the intermediate transfer belt therebetween on a downstream side of the intermediate transfer belt in a rotational direction.
In this image forming apparatus, in general, the secondary transfer outer roller is brought into abutment against the intermediate transfer belt during secondary transfer, and the secondary transfer outer roller is separated from the intermediate transfer belt during periods other than the printing operation. This enhances an inserting and extracting property of a sheet (recording material) conveyance unit during jam processing and apparatus maintenance, and hence prevents damages to the intermediate transfer belt caused by paper jam.
In most cases, in order to stably obtain intended image density, this image forming apparatus has a configuration in which a plurality of kinds of detection patterns (correction patches) to be used for adjusting density and correcting color registration are formed on the intermediate transfer belt, and the density is corrected by detecting the patterns with a sensor. An image forming apparatus disclosed in Japanese Patent Application Laid-Open No. 2012-198496 is configured as described below in order to prevent the secondary transfer outer roller from being contaminated with the detection patterns when the detection patterns are formed on the intermediate transfer belt between sheets (between recording materials to be conveyed) during printing. Specifically, the secondary transfer outer roller is separated from the intermediate transfer belt while the detection patterns are passing through the secondary transfer portion so as to prevent the secondary transfer outer roller from being contaminated with the detection patterns.
However, in the configuration in which the contamination of the secondary transfer outer roller is prevented by separating the secondary transfer outer roller from the intermediate transfer belt as described in Japanese Patent Application Laid-Open No. 2012-198496, a load change, which is caused when the separated secondary transfer outer roller is again brought into abutment against the intermediate transfer belt, may cause the following adverse effect. That is, the load change may propagate to the photosensitive drum or the intermediate transfer belt during image formation to cause an image defect such as an image streak due to shock.
The above-mentioned problem can be solved by starting image formation after the completion of an operation of bringing the second transfer outer roller into abutment against the intermediate transfer belt. However, in this case, the productivity is degraded due to the delay of the start of image formation. Further, there is also considered a method of reducing a load change by decreasing a moving speed uniformly at a time when the secondary transfer outer roller is brought into abutment against the intermediate transfer belt. However, in this case, the time period required for bringing the separated secondary transfer outer roller into abutment against the intermediate transfer belt becomes long at all times, resulting in difficulty in preventing degradation in productivity.